In a gas turbine engine such as used for aircraft applications, air is drawn into the front of the engine, compressed by a compressor, and mixed with fuel. The compressed mixture is burned in a combustor, and the hot combustion gases flow through a turbine that turns the compressor. The hot gases then flow from the rear of the engine.
The turbine includes stationary turbine vanes that deflect the hot gas flow sideways, and turbine blades mounted on a turbine wheel that turns as a result of the impingement of the hot gas stream. The turbine vanes and blades experience extreme conditions of high temperature, thermal cycling when the engine is turned on and off, oxidation, corrosion, and, in the case of the turbine blades, high stress and fatigue loadings. The higher the temperature of the hot combustion gas, the greater the efficiency of the engine. There is therefore an incentive to push the materials of the engine to ever-higher temperatures and loadings.
Nickel-based superalloys are widely used as the materials of construction of gas turbine blades and vanes. These superalloys contain primarily nickel, and a variety of alloying elements such as cobalt and aluminum, as well as refractory elements such as tantalum, tungsten, chromium, rhenium, hafnium, and others in varying amounts carefully selected to provide good mechanical properties and physical characteristics over the extremes of operating conditions experienced by the engine. However, these refractory elements, which provide the nickel-based superalloys with superior mechanical properties, also make superalloy articles susceptible to the formation of a secondary reaction zone (“SRZ”) in certain circumstances. In particular, gas turbine alloy airfoils, such as the turbine blade and vanes discussed above, typically require an aluminide coating treatment as part of a thermal barrier coating system and/or to provide environmental protection. Nickel-based superalloy articles that include refractory elements and which undergo aluminiding treatments are particularly susceptible to formation of an SRZ, wherein an acicular topologically close-packed (TCP) phase forms, such as disclosed in “A New Type of Microstructural Instability in Superalloys—SRZ,” Superalloys, 1996 by W. S. Walston, J. C. Schaeffer and W. H. Murphy, ed. R. D. Kissinger, et al. TMS pp. 9-18. Within the SRZ, the TCP phases are brittle and contain a high percentage of refractory elements. In particular, the presence of the brittle phases, the formation of high angle grain boundaries between the SRZ and the alloy, and to a lesser extent, the depletion of the refractory elements weaken the SRZ, making the SRZ essentially non-load-bearing. Because this portion of the article is unable to sustain its share of the load, the applied load is shifted to the remainder of the article, increasing the stress in this portion of the article and shortening its service life.
The problem with refractory elements in nickel-based superalloy articles forming SRZ is known, having been identified in U.S. Pat. No. 5,334,262, entitled SUBSTRATE STABILIZATION OF DIFFUSION ALUMINIDE COATED NICKEL-BASE SUPERALLOYS issued Aug. 2, 1994 to Schaeffer and assigned to the assignee of the present invention. This patent also identifies forming carbide precipitates which reduce the driving force for the formation of TCP phases within the substrate, a method for avoiding the formation of SRZ, by depositing a layer of carbon on the surface of the substrate by chemical vapor deposition and diffusing the carbon onto the surface. The presence of the carbon allows for the combination of carbon with the refractory elements to form stable carbides, substantially reducing the refractory elements unavailable for the formation of TCP phases. This patent, U.S. Pat. No. 5,334,262 is incorporated in its entirety herein by reference, forming a part of this specification.
Carbon can be introduced into the nickel-based superalloy article by carburizing techniques, such as vacuum carburizing. Vacuum carburizing of steel is a well-known technique. U.S. Pat. No. 4,836,864 issued Jun. 6, 1989, entitled “Method of Gas Carburizing and Hardening” discloses gas carburizing and hardening a steel article in a carburizing atmosphere at atmospheric pressure, heating the article in a vacuum for a predetermined period of time and hardening the article. U.S. Pat. No. 5,702,540 issued Dec. 30, 1997 entitled “Vacuum Carburizing Method and Device, and Carburized Products” teaches vacuum carburizing steel workpieces in a vacuum furnace by introducing acetylene gas into the chamber at a vacuum of 1 kPa or less to produce a hardened and uniform case depth in the steel article. U.S. patent No. Feb. 13, 2001 entitled “Vacuum Carburizing Method” divulges an improved vacuum carburizing method for steel by heating the steel material to about 900-1100° C. and then introducing ethylene gas while maintaining a vacuum of 1-10 kPa, thereby eliminating the potentially explosive acetylene and replacing the expensing vacuum pumps or mechanical booster pumps required to maintain vacuums at or below 1 kPa.
Of course, it may also be desirable to prevent selected portions of the article from being carburized by preventing contact of the surface with carbon. It is known to mask all or a selected portion of an article surface with a cover or coating to prevent it from being carburized. These coatings or covers, also referred to as a maskant, typically are platings and are usually very effective. These coatings, however, must be easy to remove or must be incorporated into the article. Typical maskants include nickel plating and copper plating. However, such plating may be unsuitable for articles that have precise shapes or include intricate details, since removing such plating after carburization can be difficult or impossible without damaging the article. However, a boron glass coating used as a maskant containing magnesium silicon compounds may be acceptable for use on intricate articles such as turbine blades, as this material can provide protection from carburization to selected, intricate areas of an airfoil, yet can be removed without damaging the airfoil. This maskant system is described in U.S. Patent Application No. 20020020471, published Feb. 21, 2002, and also is incorporated herein by reference.
Coatings typically are formed on the surfaces of the superalloy articles to protect the article from degradation in harsh, high temperature environments. One type of coating is an aluminide coating. Aluminum is diffused into the surface of the nickel-based superalloy article to form a nickel-aluminide layer, which then oxidizes to form an aluminum oxide surface coating during treatment or in service. (Optionally, noble metals such as platinum may also be diffused into the surface). The aluminum oxide surface coating renders the coated article more resistant to oxidation and corrosion, desirably without impairing its mechanical properties. Aluminide coating of turbine blades and vanes is well known and widely practiced in the industry, and is described, for example, in U.S. Pat. Nos. 3,415,672 and 3,540,878.
Recently it has been observed that, when some advanced nickel-based superalloys are coated with an aluminide coating and then exposed to service or simulated-service conditions, a secondary reaction zone (SRZ) forms in the underlying superalloy. This SRZ region is observed at a depth of from about 50 to about 250 micrometers (about 0.002-0.010 inches) below the original superalloy surface that has received the aluminide coating. The presence of the SRZ reduces the mechanical properties in the affected region, because the material in the SRZ appears to be brittle and weak, and forms a high angle grain boundary between SRZ and the alloy.
The formation of the SRZ is a major problem in some types of turbine components, because there are cooling channels located about 750 micrometers (about 0.030 inches) below the surface of the article. Cooling air is forced through the channels during operation of the engine, to cool the structure. If the SRZ forms in the region between the surface and the cooling channel, it significantly weakens that region and can lead to reduced strength and fatigue resistance of the article.
While the prior art prevents the formation of the TCP phases that weaken the SRZ, the prior art relies solely on a diffusion mechanism to diffuse inward the carbon deposited on the surface of the superalloy substrate. While acceptable results can thus be obtained, it is desirable to improve the methods of deposition to control the depth of carburization while allowing the absorption of carbon into the surface quickly.